This invention relates to a process for the fractionation of a gaseous mixture comprising hydrogen, methane, and carbon monoxide, with a methane scrubber operating at a low temperature.
In a conventional process of this type (Hydrocarbon Processing, November 1971, p. 164), whose details are incorporated by reference herein, the gaseous mixture, obtained by steam reforming of hydrocarbons and subsequent CO.sub.2 separation, is cooled to a temperature of about 90.degree. K. and subjected to a scrubbing operation with liquid methane. During this step, substantially CO-free hydrogen is obtained as the top product of the scrubbing column, whereas the carbon monoxide dissolved in the methane is withdrawn from the bottom of the column. The loaded scrubbing medium (methane) is thereafter regenerated in a CO/CH.sub.4 distillation column and reintroduced into the scrubbing column. In the distillation column, carbon monoxide is obtained as the top product and it represents a further product of the process. The refrigeration required for regenerating the scrubbing medium is made available by a cycle wherein a part of the separated carbon monoxide is circulated. For this purpose, the carbon monoxide is compressed in a compressor, condensed in the bottom of the regenerating column (i.e., CO/CH.sub.4 separating column), thereby heating up the bottom, expanded, and reintroduced into the regenerating column as a recycle stream.
The methane obtained in this conventional process in the bottom of the regenerating column is, insofar as it exceeds the requirements of the methane scrubbing column, vaporized and removed as a byproduct gas together with gases obtained at other locations in the process.
The conventional process is advantageously employed for the fractionation of a gaseous mixture having a relatively low methane content, usually below 5%. In this process, the separated methane can be utilized to a large extent to compensate for scrubbing medium losses. However, if a gaseous mixture having a higher methane content is to be fractionated, then the consequence is a large additional expenditure for the CO/CH.sub.4 separation in the regenerating column and thus a greatly increased expense for the carbon monoxide refrigeration cycle.